Detailed Description
8500C/8500C+ System Maintenance Manual 4-9
b. Display Heater. The filtered +27 VDC power from the power supply filter is regulated at
+12 VDC and applied to the heater controller module. This module samples the resistance
of the heater every 10 seconds. If the resistance falls below approximately 83 ohms,
indicating that the screen temperature is below approximately 7°C, then the controller
begins heating the display. As the heater temperature rises, the heater resistance increases.
Once the resistance rises above approximately 83 ohms, the screen is at operating
temperature and the heater is turned off. The heater is capable of keeping the LCD screen
at +7°C with ambient temperatures down to -40°C.
c. EL Controller
. The filtered +27 VDC power from the power supply is applied to a variable
voltage regulator. The voltage regulator output (+1.5 to +13 VDC) is changed by an
external BRIGHTNESS ADJ potentiometer on the control panel in the disk drive
compartment. This regulated voltage drives an 800 Hz, 100 Vrms inverter, which powers
the EL lamps behind the LCD and keypanel. The brightness of the lamps is proportional to
the inverter AC voltage. The inverter AC voltage output is proportional to the DC voltage
input to the inverter.
4.2.1.2 Printer/Disk Drive Assembly
The printer/disk drive assembly contains three printed circuit boards: a printer board, a distribution
board, and a power supply board; a printer assembly; and a disk drive assembly. The printer board
drives the printer. The distribution board provides an interface between the balancer/analyzer assembly
and printer/disk drive assembly. The power supply board generates the DC operating voltages for the
balancer/analyzer assembly and printer/disk drive assembly. The printer assembly prints the data. The
disk drive assembly reads the data from the diskette.
4.2.1.2.1 Printer Board
The printer board receives data and control commands from the communications controller in the
8500C/C+ digital board, and status signals are returned to the digital board. The data and control
commands are converted from serial form to parallel form to drive the print head. Three amplifiers
amplify the print drive signals to the proper levels needed to drive the print heads. The board also
contains a paper feed switch to index paper through the paper feed slot. An LED paper sensor informs
the processor when paper is in the unit.
4.2.1.2.2 Distribution Board
The distribution board provides cable interfacing between the balancer/analyzer assembly and printer/
disk drive assembly.
4.2.1.2.3 Power Supply Board (Figure 4-5)
The power supply board converts the 28 VDC input power from the aircraft to the DC voltages needed
to operate the circuit boards in the balancer/analyzer assembly and printer/disk drive assembly. The
input power from the aircraft can be 12 to 28 VDC of either polarity. The input power is applied to a
diode bridge to set the input voltage to the correct polarity. The output of the bridge is applied across a
FET switch and transformer. The transformer outputs are rectified and filtered to produce two +5
VDC, one +12 VDC, and one -12 VDC output voltage. The power supply contains a closed-loop
regulator, which keeps the output voltages constant for varying input voltage and load. An optoisolator
feeds back the output voltage across the isolation barrier. A light emitting diode (LED) indicates when
power is on.
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